Bipolar transistors are generally constructed from two pn-junctions lying close together in a semiconductor crystal. In different configurations, either two n-doped regions are separated from one another by a p-doped region (npn transistors) or two p-doped regions by an n-doped region (pnp transistors). The three differently doped regions are referred to as the emitter, the base, and the collector. Therefore, a bipolar transistor is essentially a three terminal device having three doped regions of alternating doping type.
Bipolar transistors may exhibit desirable features such as high current gain and a very high cut-off frequency for switching applications, and high power gain and power density for microwave amplifier applications. These features make bipolar transistors important components in logic circuits, communications systems, and microwave devices. As with other types of semiconductor devices, there is a demand for bipolar transistors having increasingly higher operating frequencies and/or switching speeds. Since their invention in 1947, many attempts have been made to meet these demands and improve the performance of such transistors with respect to their speed, power, and frequency characteristics. These attempts have focused on making devices better suited for high frequency applications such as microwave and logic devices. One particular way to meet these demands for higher frequency operation is to provide a device with a low base resistance and a low base-collector capacitance.